Downlink coordinated transmission in OFDMA systems

ABSTRACT

Embodiments described herein include methods for improving the SINR, and therefore communication quality or rate, in the downlink of a cellular communication system. In an embodiment, the system is an orthogonal-frequency-division multiple-access (OFDMA) system. In an embodiment, a set of terminals is designated a coordinated-transmission group. The set of terminal is chosen such that the slot-allocations of the set are given special treatment to alleviate interference from other sectors or cells. All terminals within a coordination group generally use the same slot, but embodiments are not so limited.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/783,936, filed Mar. 20, 2006, which is incorporatedby reference in its entirety herein.

This application is also related to copending U.S. patent applicationSer. No. 11/725,920, entitled “Downlink Coordinated Transmission inOFDMA Systems, Including WiMax Systems”, filed Mar. 20, 2007, which isincorporated by reference in its entirety herein.

TECHNICAL FIELD

The disclosed embodiments relate to wireless cellular communicationssystems and methods.

BACKGROUND OF THE DISCLOSURE

Cellular telephone users (cellular telephone users and cellulartelephones are also referred to herein as “terminals”) in the downlink(base station to terminal) path of a cellular system who are on or nearthe boundaries between cells or sectors usually have lowsignal-to-interference ratio (SINR) because of the strong interferencefrom neighboring cells or sectors. These terminals often have poorconnectivity with their respective base stations as a result.

FIG. 1 is a block diagram of a prior art cellular telephone system. FIG.1 includes three cells, labeled A, B, C, in a typical arrangement. Eachcell includes three respective sectors labeled 1, 2, and 3. The cell andsector boundaries are indicated by lines. Each sector typically servesits own set of terminals.

Typically the edge of a cell represents a boundary where the signalstrength from a serving base station A (not shown) is comparable to thestrength from one or more neighboring base stations (base station B, C,and so on, not shown). The transmissions from base station B, in contactwith its own terminals, may then appear as interference to the terminalon the boundary, thus causing low SINR and impairing the data rate tothe terminal.

In a similar fashion, the edge of a sector represents a boundary wherethe transmission beam-pattern allows transmissions from one sector tospill over into another sector as interference. Thus, a terminal beingserved in sector 1 may also see interfering transmissions from sectors 2and 3. In an orthogonal-frequency-division multiple-access (OFDMA)system, time and frequency are usually divided into sub-units calledsymbols (in time t) and subcarriers (in frequency k). OFDMA systems arealso referred to as orthogonal-frequency-duplex multiple-access systems.The base station typically assigns multiple time symbols and subcarriersto carry the data from the base station to the terminals. Each terminalwithin a sector is usually assigned a distinct subset of the availablesymbols and subcarriers. Such subsets of symbols and subcarriers aredenoted “slots” herein. The number of subcarriers, the number ofsymbols, and the level of modulation (which is usually a function of theSINR) determines the data rate to the terminal.

In a “frequency re-use one” system, every cell and sector is free toutilize all of the subcarriers and symbols without regard for thesubcarriers and symbols used in other cells or sectors. Such a systemtherefore has significant interference between sectors and cells,especially on their boundaries. In systems with higher frequency re-use,the interference tends to be less on sector and cell boundaries but canappear elsewhere.

One way of expressing the effect of interference in cellularcommunication systems is provided by Equations 1-3 below. In thisexample, Base station A transmits on subcarrier k and time t on Sector 1the signals_(k,t) ⁽¹⁾=u_(k,t) ⁽¹⁾  Equation 1where u_(k,t) ⁽¹⁾ is a unit-energy data-symbol intended for a terminalin Sector 1. Base station A transmits on the same subcarrier k and timet on Sector 2 the signals_(k,t) ⁽²⁾=u_(k,t) ⁽²⁾  Equation 2where u_(k,t) ⁽²⁾ is a data-symbol intended for a distinct terminal inSector 2. The terminal in Sector 1 at time t and subcarrier k receivesr _(k,t) =h _(k,t) u _(k,t) ⁽¹⁾ +g _(k,t) u _(k,t) ⁽²⁾ +w_(k,t)  Equation 3

where the channel between the terminal and its antenna (in Sector 1) ish_(k,t) and g_(k,t). The terminal might have trouble decoding itsintended message u_(k,t) ⁽¹⁾ if the channel magnitudes |g_(k,t)| and|h_(k,t)| are comparable.

Incorporation by Reference

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art cellular telephone system.

FIG. 2 is a diagram illustrating a time-frequency schedule for aninterference killing scheme according to an embodiment.

FIG. 3 is a diagram illustrating a time-frequency schedule for aco-transmission scheme according to an embodiment.

FIG. 4 is a block diagram illustrating the benefit of adding beamformingto aid a terminal according to an embodiment.

FIG. 5A is a diagram illustrating a time-frequency schedule for acoordinated transmission zone scheme according to an embodiment.

FIG. 5B is a diagram illustrating a time-frequency schedule for acoordinated transmission zone scheme according to an embodiment.

DETAILED DESCRIPTION

Embodiments described herein include methods for improving the SINR, andtherefore communication quality or rate, in the downlink of a cellularcommunication system. In an embodiment, the system is anorthogonal-frequency-division multiple-access (OFDMA) system. In anembodiment, a set of terminals is designated a coordinated-transmissiongroup (also referred to herein as a “coordination group”). The set ofterminal is chosen such that the slot-allocations of the set are givenspecial treatment to alleviate interference from other sectors or cells.All terminals within a coordination group generally use the same slot,but embodiments are not so limited. Various embodiments are especiallysuitable for use in WiMax communication systems. A WiMax communicationsystem conforms to a WiMax standard, such as IEEE 802.16, for example.The standard defines “zones” where terminals are scheduled according totheir requirements for downlink traffic. As disclosed herein, zones are“repurposed” to handle the coordinated-transmission, although the zonewas originally intended for another purpose. Various defined zonesinclude dedicated pilots used by the terminals to learn the channel. Inan embodiment, the transmission protocol is changed as required to alignit across sectors or cells. Once this alignment (for example intime-frequency and pseudo-random sequence) is established, a basestation can use one sector in the zone to augment the transmission inanother sector in the zone.

Boundary terminals are on the boundary of a sector and/or a cell.Boundary terminals are thus usually relatively highly distressed, thatis they suffer from greater interference than non-boundary terminals.Distressed terminals may have an SINR so low that they are not able tomeet the minimum communication rate for the system. Boundary terminalsare typically members of the coordination group. However, non-distressed“sacrificial” terminals may also be included in the coordination group.“Sacrificial terminals” as used herein includes terminals that areplaced in a coordination group even though they have high SINR. Acoordination group is handled in many ways, according to embodimentsdisclosed herein, to improve SINR for a distressed terminal.

According to an embodiment, a power push-pull technique facilitatestransmission for distressed terminals at the expense of transmission fornon-distressed terminals. Each sector transmits data to its own terminalin a defined slot, and one low SINR (distressed) terminal is paired withone or more sacrificial terminals in the slot. The transmit power forthe distressed terminal is boosted and, simultaneously, power for thesacrificial terminal(s) is reduced. The distressed terminal effectivelyattains better SINR because of “push” from the higher power and the“pull” from the lower interference. On the other hand, the sacrificialterminal loses some SINR. However, the sacrificial terminal can stillmaintain a desired minimum acceptable level or quality of service.

FIG. 2 is a diagram illustrating a time-frequency schedule for aninterference killing scheme of another embodiment in which interferenceaffecting distressed terminals is killed. For example, only the sectorcontaining a distressed terminal transmits in the defined slot. Theother sectors shut down transmission. This immediately improves thedistressed terminal's SINR because significant interference is removed.As shown in FIG. 2, the upper left time-frequency slot is used by basestation A and a distressed terminal (referred to as a “user”) in sector1. This slot is not used by sector 2. The lower left time-frequency slotis used by base station A and a distressed terminal in sector 2. Thisslot is not used by sector 1. To the right of the previously describedslots there are slots for other transmissions and for base station B insector 3. The diagram shows that there is no interference for theterminal in sector 1 or the terminal in sector 2.

In another embodiment, a coordination group contains only one, generallydistressed, terminal and more than one base station transmits the samesignal. According to this co-transmission method, the terminal receivesmultiple superimposed copies of the signal, and thus benefits fromadditional power and certain forms of diversity. Interference is greatlyreduced. Each base station may weight its transmission by a scalingfactor to assist the terminal in reception.

Equations 4-6 further illustrate co-transmission. Consider as an examplethat Base station A transmits on subcarrier k and time t on Sector 1s _(k,t) ⁽¹⁾ =h _(k,t) *u _(k,t) ⁽¹⁾  Equation 4where u_(k,t) ⁽¹⁾ is a unit-energy data-symbol intended for a terminalin Sector 1. Base station A transmits on the same subcarrier k and timet in Sector 2s _(k,t) ⁽²⁾ =g _(k,t) *u _(k,t) ⁽¹⁾  Equation 5where g_(k,t) is the channel between Sector 2 and the terminal. Theterminal then receives a coherent combination of the two transmissionsr _(k,t)=(|h _(k,t)|² +|g _(k,t)|²)u _(k,t) ⁽¹⁾ +w _(k,t)  Equation 6where w_(k,t) is additive interference (from other cells) and noise.Alternatively, the Sector 1 transmission could be normalized to unity,for example using h_(k,t)/|h_(k,t)|, and similarly for Sector 2. Thisconstitutes a form of coherent transmission and requires the downlinkchannel to be known by the base station.

FIG. 3 is a diagram illustrating a time-frequency schedule for aco-transmission scheme. Base station A is using the same upper lefttime-frequency slots to transmit data (on sector 1 and sector 2) to theterminal on the sector boundary.

In a co-transmission scenario in which the base station simply transmitsthe data on the two sectors without pre-weighting, the terminal receivesr _(k,t)=(h _(k,t) +g _(k,t))u _(k,t) ⁽¹⁾ +w _(k,t)  Equation 7

This method does not require the base station to know the downlinkchannel.

In a time-division duplex (TDD) system, the uplink (from terminal tobase station) and downlink transmissions are slotted in time and occupythe same carrier frequency band. The transmissions are usuallyalternating: first downlink then uplink then downlink again, and so onin some predefined arrangement. Because the same frequency bands areused, there is often information in the uplink that can be used todetermine which terminals suffer from significant interference fromadjacent cells or sectors. The terminal can also supply interferencemeasurements or channel quality indicators or use its knowledge of basestation pilots (sometimes referred to as the “active set”) to determinethe relation of its primary channel to the source(s) of interference.

In a frequency-division duplex (FDD) system, the uplink and downlinktransmissions use different carrier frequency bands, usually separatedfar enough apart that measurements in the uplink cannot directly be usedon the downlink to infer the quality of channels seen by the terminaland determine the coordination group. In this case, the terminal reportsall information to the base station about the nature and source ofinterference, including, but not limited to, its active set.

In another embodiment, distressed and sacrificial terminals areidentified. The base station uses terminal reports of signal strengthand SINR observed from different base stations and sectors to determinesuitable candidates for the coordination group.

In an embodiment, pseudo-random sequences are aligned. Sectors and cellsare often identified and protected by unique pseudo-random bit sequences(PRBSs) either applied to pilots in frequency or time. Thus, to obtainthe advantage of coordinated transmission, the sectors or cells arecoordinated such that their sequences are the same for the coordinationgroup. In an embodiment, this is accomplished by assigning atime-frequency slot during which the coordination group is handled usinga separate transmission protocol with the same PRBS.

In embodiments, various methods previously described are augmented withbeamforming using multiple antennas. The multiple antennas may beavailable locally within a sector or cell. If the multiple antennas arenot available locally, they can also be available from across multiplesectors or cells. For example, in an embodiment, the power push-pullmethod previously described is employed, and a beam is steered toward adistressed terminal in the serving sector and away from the distressedterminal in the interfering sector.

In another embodiment employing a co-transmission method as previouslydescribed, beams from two different sectors (or cells) are coordinatedto the distressed terminal. FIG. 4 is a block diagram illustrating thebenefit of adding beamforming to aid a terminal on the Sector 2-3boundary. Terminal 402 is on the Sector 2-Sector 3 boundary, and is adistressed terminal. However, beam 404 and beam 406 from Sector 3 andSector 2 are both directed toward terminal 402 to provide improvedservice.

In an embodiment, beamforming is augmented with spatial multiplexing.For example, in addition to the previously described multiple antennabeamforming, multiple terminals in the same coordination group arehandled using simultaneous orthogonal or quasi-orthogonal beams. In suchan embodiment, the terminals do not interfere significantly with eachother. An embodiment uses a form of spatial multiplexing in whichterminals are distinguished based on their spatial signatures.

Methods previously described are further augmented in embodiments withspace-time coding using multiple antennas. As used herein, space-timecoding refers to a method of encoding the data, potentially usingredundancy, and sending the resulting symbols of this code using theavailable antennas. The multiple antennas can be from across multiplesectors or cells if they are not available locally, or within a sectoror cell if they are available locally.

In various embodiments, pilot subcarriers (from which the transmissionchannel is usually learned at the terminal) are treated in the same wayas data subcarriers.

Various embodiments are particularly applicable to systems in compliancewith a WiMax standard, such as IEEE 802.16. These embodiments implementcoordinated transmission within the WiMax standard in various ways. TheWiMax standard defines “zones” where terminals are scheduled accordingto their requirements for downlink traffic. In various embodiments,WiMax-defined zones are “repurposed” to handle thecoordinated-transmission group. One such zone that is repurposed tohandle the coordinated-transmission group is the so-called Band-AMC(adaptive channel and modulation) Beamforming Zone. This zone wasoriginally intended for another purpose. One interesting feature of thiszone is that there are dedicated pilots used by the terminals to learnthe channel. The presence of the pilots allows the transmission protocolto be changed across sectors or cells without adverse effects. In anembodiment, a WiMax-defined zone is repurposed as a “coordinatedtransmission zone” in which inter-sector or inter-cell coordinatedtransmission may be used.

As an example for the purpose of illustrating an embodiment, suppose aterminal being served by Base station A, Sector 1 is on the boundarywith Sector 2 and it is desirable to create a coordinated transmissionzone to put this terminal in a coordination group (as previouslydescribed). A time-frequency slot is set aside, and a Band-AMCBeamforming Zone that uses this slot is specified in Sectors 1 and 2.

Sector 2 normally randomizes its transmissions on pilots and datasubcarriers with a pseudo-random sequence that is distinct from thesequence used in Sector 1. To create a coordinated transmission zone,the pseudo-random sequence used in Sector 2 is changed to coincide withthe sequence in Sector 1 in this time-frequency slot. Once thisalignment in time-frequency and pseudo-random sequence is established,the base station can use Sector 2 to augment the transmission in Sector1 using various methods previously described herein. Because of thepresence of the dedicated pilots, the terminal is not aware of the basestation behavior and decodes the pilots and data in this zone normally.

FIG. 5A is a diagram illustrating a time-frequency schedule for acoordinated transmission zone scheme. Sector 1 transmission is shownaccording to an embodiment. A Band-AMC beamforming zone occupiestime-frequency slots as shown. The beamforming zone is used as acoordinated transmission zone. At the top of this coordinatedtransmission zone is a slot for a terminal in Sector 1 on the Sector 2boundary, and using PRBS 1. Immediately below this slot is a slotoccupied by other terminals in Sector 1 using PRBS 1. Below this slot,and at the bottom of the beamforming zone, is a slot using PRBS 2 orPRBS 3.

FIG. 5B is a diagram illustrating another time-frequency schedule for acoordinated transmission zone scheme. Sector 2 transmission is shownaccording to an embodiment. A Band-AMC beamforming zone occupiestime-frequency slots as shown. The beamforming zone is used as acoordinated transmission zone. At the top of this coordinatedtransmission zone is a slot for a terminal in Sector 1 on the Sector 2boundary, and using PRBS 1. Immediately below this slot is a slotoccupied by terminals in Sector 2 using PRBS 2. Below this slot, and atthe bottom of the beamforming zone, is a slot using PRBS 3.

In various embodiments, the pseudo-random sequences that are normallydifferent between sectors and cells in a WiMax system are made to be thesame within certain sectors or cells and certain time-frequency slots tocreate coordinated transmission zones within any zone having dedicatedpilot channels. For example, as just described, the Band-AMC BeamformingZone is adapted to implement a coordinated transmission zone in a systemwith dedicated pilots, such as a WiMax system. In another embodiment,the Partially-Used Subcarrier Beamforming Zone (PUSC-BF) is used toimplement a coordinated transmission zone in a system with dedicatedpilots, such as a WiMax system. The PUSC implementation ofcoordinated-transmission is similar to the Band-AMC implementation, witha difference being in the way the subcarriers are categorized into majorgroups and assigned to terminals.

In yet another embodiment, the Multicast-Broadcast Service Zone (MBS) isused to implement a coordinated transmission zone in a WiMax system. TheWiMax standard specifies that, in an MBS zone, the pseudo-randomsequence across all sectors and cells must be identical to allowbroadcast transmission. Rather than transmit broadcast,coordinated-transmission is used in this zone.

Downlink coordinated transmission in OFDMA systems is described herein,including a cellular communication method, the method comprising:identifying distressed terminals of a plurality of terminals in acellular network; and coordinating downlink transmission to theplurality of terminals based on the identified distressed terminals,wherein the network comprises cells, and each cell comprises sectors.

In an embodiment, coordinating downlink transmission comprises:designating the plurality of terminals as a group, wherein a terminalcomprises a cellular communication device; and handling communicationsinvolving a terminal in the group in a preferential manner.

In an embodiment, the cellular communication method comprises anorthogonal-frequency-division multiple-access (OFDMA) method.

In an embodiment, coordinating downlink transmissions comprisespreferentially selecting slot allocations of a terminal, wherein a slotcomprises a subset of available symbols and available subcarriers in asector of the terminal.

In an embodiment, the plurality of terminals in the group use a sameslot, wherein a slot comprises a subset of available symbols andavailable subcarriers in a sector of a terminal.

In an embodiment, the plurality of terminals comprises boundaryterminals that are on one or more of a sector boundary and a cellboundary.

In an embodiment, the plurality of terminals comprises terminals with arelatively low signal-to-interference ratio (SINR) with respect to otherterminals of the plurality of terminals.

In an embodiment, the plurality of terminals comprises: distressedterminals with a low SINR relative to sacrificial terminals; andsacrificial terminals with a high SINR relative to distressed terminals.

In an embodiment, the plurality of terminals comprises distressedterminals with a low SINR relative to sacrificial terminals, andsacrificial terminals with a high SINR relative to distressed terminals,the method further comprising: a sector transmitting data to a terminalin an allocated slot; and pairing a distressed terminal with one or moresacrificial terminals such that transmit power for the distressedterminal is increased while transmit power for the sacrificial terminalis decreased.

In an embodiment, the plurality of terminals comprises distressedterminals with a low SINR relative to sacrificial terminals, andsacrificial terminals with a high SINR relative to distressed terminals,the method further comprising selecting slot allocations of a sectorcomprising a distressed terminal to correspond with slots in which othersectors are not transmitting.

In an embodiment, the plurality of terminals comprises a single terminalthat has a relatively low SINR compared to other terminals of theplurality of terminals, the method further comprising: multiple basestations transmitting a same signal to the single terminal; and thesingle terminal receiving copies of the same signal from each of themultiple base stations.

In an embodiment, each of the multiple base stations weights itstransmission by a scaling factor.

In an embodiment, designating the plurality of terminals comprises:receiving base station reports from multiple sectors, wherein thereports comprise one or more of signal strength and SINR for terminalsin the sectors; and analyzing the reports to determine terminals todesignate.

In an embodiment, the plurality of terminals is distributed acrossmultiple sectors and multiple cells, the method further comprisingaligning pseudo-random bit sequences (PRBS) of the multiple sectors andmultiple cells.

In an embodiment, aligning comprises assigning a time-frequency slotduring which the plurality of terminals is handled using a separatetransmission protocol with a same PRBS.

An embodiment further comprises: steering a beam toward a distressedterminal in a serving sector; and steering a beam away from a distressedterminal in an interfering sector.

In an embodiment, steering comprises beamforming using multiple antennasfrom at least one of a sector and a cell.

In an embodiment, steering comprises beamforming using multiple antennasfrom across at least one of one or more sectors and one or more cells.

An embodiment further comprises: designating the distressed terminal inthe serving sector and the distressed terminal in the interfering sectoras being in the plurality if terminals in the group; and usingsimultaneous orthogonal beams to reduce interference between thedistressed terminal in the serving sector and the distressed terminal inthe interfering sector.

An embodiment further comprises: designating the distressed terminal inthe serving sector and the distressed terminal in the interfering sectoras being in the plurality if terminals in the group; and usingsimultaneous quasi-orthogonal beams to reduce interference between thedistressed terminal in the serving sector and the distressed terminal inthe interfering sector.

An embodiment further comprises coordinating beams from two differentsectors to the single terminal.

An embodiment further comprises coordinating beams from two differentcells to the single terminal.

An embodiment further comprises performing space-time coding of the datato be transmitted; and transmitting the coded data using availableantennas, wherein the available antennas comprise multiple antennas fromat least one of a sector and a cell.

An embodiment further comprises performing space-time coding of the datato be transmitted; and transmitting the coded data using availableantennas, wherein the available antennas comprise multiple antennas fromat least one of one or more sectors and one or more cells.

In an embodiment, handling communications comprises handling pilotsubcarriers in a same manner as data subcarriers.

Downlink coordinated transmission in OFDMA systems as described hereinfurther include an orthogonal-frequency-division multiple-access (OFDMA)communication method, comprising: identifying distressed terminals of aplurality of terminals in a cellular network; and coordinating downlinktransmission to the plurality of terminals based on the identifieddistressed terminals, wherein the network comprises cells, and each cellcomprises sectors.

In an embodiment, coordinating downlink transmission comprises:designating the plurality of terminals as a group, wherein a terminalcomprises a cellular communication device; and handling communicationsinvolving a terminal in the group in a preferential manner.

In an embodiment, the plurality of terminals in the group will use asame slot, wherein a slot comprises a subset of available symbols andavailable subcarriers in a sector of a terminal.

In an embodiment, the plurality of terminals comprises boundaryterminals that are on one or more of a sector boundary and a cellboundary.

In an embodiment, wherein the plurality of terminals comprises terminalswith a relatively low signal-to-interference ratio (SINR).

In an embodiment, the plurality of terminals comprises: distressedterminals with a low SINR relative to a sacrificial terminal; andsacrificial terminals with a high SINR relative to a distressedterminal.

Downlink coordinated transmission in OFDMA systems as described hereinfurther include a computer-readable medium having stored thereoninstruction, that when executed in a cellular communication system,cause a communication method to be performed, the method comprising:

identifying distressed terminals of a plurality of terminals in acellular network; and

coordinating downlink transmission to the plurality of terminals basedon the identified distressed terminals, wherein the network comprisescells, and each cell comprises sectors.

Downlink coordinated transmission in OFDMA systems as described hereinfurther include an orthogonal-frequency-division multiple-access (OFDMA)cellular communication method: identifying distressed terminals of aplurality of terminals in a cellular network; and coordinating downlinktransmission to the plurality of terminals based on the identifieddistressed terminals, comprising designating a time-frequency slot forassisting the identified distressed terminals.

An embodiment further comprises specifying a transmission zone that usesthe time-frequency slot, wherein terminals are scheduled in thetransmission zone according to their respective requirements fordownlink traffic.

In an embodiment, the transmission zone comprises a dedicated pilot usedby terminals to learn a channel.

An embodiment further comprises changing one or more pseudo-random bitsequences (PRBSs) used in one or more of multiple sectors in thetransmission zone such that the PRBSs of the multiple sectors coincide.

An embodiment further comprises using one sector in the transmissionzone to augment transmission in another sector in the transmission zone.

In an embodiment, the transmission zone is an adaptive channel andmodulation (AMC) beamforming zone.

In an embodiment, the transmission zone is a partially-used subcarrierbeamforming zone (PUSC-BF).

In an embodiment, the transmission zone is a multicast-broadcast servicezone (MBS).

In an embodiment, the transmission zone allows at least one ofinter-sector and inter-cell coordinated transmission.

In an embodiment, the distressed terminals comprise terminals with arelatively low SINR compared to other terminals of the plurality ofterminal.

An embodiment further comprises: a sector transmitting data to aterminal in a designated slot; and pairing a distressed terminal withone or more sacrificial terminals such that transmit power for thedistressed terminal is increased while transmit power for thesacrificial terminal is decreased.

In an embodiment, the plurality of terminals comprises a singledistressed terminal, the method further comprising: multiple basestations transmitting a same signal to the single distressed terminal;and the single distressed terminal receiving a copy of the signal fromeach of the multiple base stations.

In an embodiment, each of the multiple base stations weights itstransmission by a scaling factor.

In an embodiment, identifying distressed terminals comprises: receivingbase station reports from multiple sectors, wherein the reports compriseone or more of signal strength and SINR for terminals in the sectors;and analyzing the reports to determine distressed terminals.

An embodiment further comprises: steering a beam toward a distressedterminal in a serving sector; and steering a beam away from a distressedterminal in an interfering sector.

In an embodiment, steering comprises beamforming using multiple antennasfrom at least one of a sector and a cell.

In an embodiment, steering comprises beamforming using multiple antennasfrom across at least one of one or more sectors and one or more cells.

An embodiment further comprises: using simultaneous orthogonal beams toreduce interference between the distressed terminal in the servingsector and the distressed terminal in the interfering sector.

An embodiment further comprises using simultaneous quasi-orthogonalbeams to reduce interference between the distressed terminal in theserving sector and the distressed terminal in the interfering sector.

An embodiment further comprises coordinating beams from two differentsectors to the single terminal.

An embodiment further comprises coordinating beams from two differentcells to the single terminal.

An embodiment further comprises: performing space-time coding of thedata to be transmitted; and transmitting the coded data using availableantennas, wherein the available antennas comprise multiple antennas fromat least one of a sector and a cell.

An embodiment further comprises: performing space-time coding of thedata to be transmitted; and transmitting the coded data using availableantennas, wherein the available antennas comprise multiple antennas fromat least one of one or more sectors and one or more cells.

In an embodiment, handling communications comprises handling pilotsubcarriers in a same manner as data subcarriers.

Downlink coordinated transmission in OFDMA systems as described hereinfurther include a computer-readable medium having stored thereoninstruction, that when executed in a cellular communication system,cause an orthogonal-frequency-division multiple-access (OFDMA)communication method to be performed, the method comprising: identifyingdistressed terminals of a plurality of terminals in a cellular network;and coordinating downlink transmission to the plurality of terminalsbased on the identified distressed terminals, comprising designating atime-frequency slot for assisting the identified distressed terminals.

An embodiment further comprises specifying a transmission zone that usesthe time-frequency slot, wherein terminals are scheduled in thetransmission zone according to their respective requirements fordownlink traffic.

In an embodiment, the transmission zone comprises a dedicated pilot usedby terminals to learn a channel.

In an embodiment, the method further comprises changing one or morepseudo-random bit sequences (PRBSs) used in one or more of multiplesectors in the transmission zone such that the PRBSs of the multiplesectors coincide.

In an embodiment, the method further comprises using one sector in thetransmission zone to augment transmission in another sector in thetransmission zone.

Aspects of the methods described herein may be implemented asfunctionality programmed into any of a variety of circuitry, includingprogrammable logic devices (“PLDs”), such as field programmable gatearrays (“FPGAs”), programmable array logic (“PAL”) devices, electricallyprogrammable logic and memory devices and standard cell-based devices,as well as application specific integrated circuits. Embodiments mayalso be implemented as microcontrollers with memory (such aselectrically erasable programmable read-only memory (“EEPROM”)),embedded microprocessors, firmware, software, etc. Furthermore, aspectsmay be embodied in microprocessors having software-based circuitemulation, discrete logic (sequential and combinatorial), customdevices, fuzzy (neural) logic, quantum devices, and hybrids of any ofthe above device types. Of course the underlying device technologies maybe provided in a variety of component types, e.g., metal-oxidesemiconductor field-effect transistor (“MOSFET”) technologies likecomplementary metal-oxide semiconductor (“CMOS”), bipolar technologieslike emitter-coupled logic (“ECL”), polymer technologies (e.g.,silicon-conjugated polymer and metal-conjugated polymer-metalstructures), mixed analog and digital, etc.

The various functions disclosed herein may be described using any numberof combinations of hardware, firmware, and/or as data and/orinstructions embodied in various machine-readable or computer-readablemedia, in terms of their behavioral, register transfer, logic component,and/or other characteristics. Computer-readable media in which suchformatted data and/or instructions may be embodied include, but are notlimited to, non-volatile storage media in various forms (e.g., optical,magnetic or semiconductor storage media) and carrier waves that may beused to transfer such formatted data and/or instructions throughwireless, optical, or wired signaling media or any combination thereof.Examples of transfers of such formatted data and/or instructions bycarrier waves include, but are not limited to, transfers (uploads,downloads, e-mail, etc.) over the Internet and/or other computernetworks via one or more data transfer protocols (e.g., hypertexttransfer protocol (“HTTP”), file transfer protocol (“FTP”), simple mailtransfer protocol (“SMTP”), etc.).

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in a sense of “including,but not limited to.” Words using the singular or plural number alsoinclude the plural or singular number respectively. Additionally, thewords “herein,” “hereunder,” “above,” “below,” and words of similarimport refer to this application as a whole and not to any particularportions of this application. When the word “or” is used in reference toa list of two or more items, that word covers all of the followinginterpretations of the word: any of the items in the list; all of theitems in the list; and any combination of the items in the list.

The above description of illustrated embodiments is not intended to beexhaustive or limited by the disclosure. While specific embodiments of,and examples are described herein for illustrative purposes, variousequivalent modifications are possible, as those skilled in the relevantart will recognize. The teachings provided herein can be applied toother methods, and not only for the methods described above. Theelements and acts of the various embodiments described above can becombined to provide further embodiments. These and other changes can bemade to methods in light of the above detailed description.

In general, in the following claims, the terms used should not beconstrued to be limited to the specific embodiments disclosed in thespecification and the claims, but should be construed to include allmethods that operate under the claims. Accordingly, the methods are notlimited by the disclosure, but instead the scope is to be determinedentirely by the claims. While certain aspects are presented below incertain claim forms, the inventors contemplate the various aspects inany number of claim forms. For example, while only one aspect is recitedas embodied in a machine-readable medium, other aspects may likewise beembodied in a machine-readable medium. Accordingly, the inventorsreserve the right to add additional claims after filing the applicationto pursue such additional claim forms for other aspects as well.

1. A cellular communication method, the method comprising: identifying adistressed terminal of a plurality of terminals in a cellular network,wherein the distressed terminal is identified as receiving downlinktransmission signals having an signal to interference and noise ratio(SINR) less than a threshold; and coordinating downlink transmission tothe plurality of terminals based on at least the identified distressedterminal, wherein the network comprises cells, and each cell comprisessectors; wherein coordinating the downlink transmission comprisesmultiple base stations transmitting a same signal to the identifieddistressed terminal; and the distressed terminal receiving a copy of thesame signal from each of the multiple base stations, comprisingselecting pseudo-random bit sequences (PRBS), and aligning the PRBS ofthe multiple sectors and multiple cells, wherein the aligning comprisesassigning a time-frequency slot for the identified distressed terminal;wherein the plurality of terminals comprises distressed terminals with alow SINR relative to sacrificial terminals, and sacrificial terminalswith a high SINR relative to distressed terminals, the method furthercomprising: a sector transmitting data to a terminal in an allocatedslot; and pairing a distressed terminal with one or more sacrificialterminals such that transmit power for the distressed terminal isincreased while transmit power for the sacrificial terminal isdecreased; designating a distressed terminal in a serving sector and adistressed terminal in an interfering sector as being in the pluralityof terminals; and using beams to reduce interference between thedistressed terminal in the serving sector and the distressed terminal inthe interfering sector.
 2. The method of claim 1, wherein coordinatingdownlink transmission comprises: designating the plurality of terminalsas a group, wherein a terminal comprises a cellular communicationdevice; and handling communications involving a terminal in the group ina preferential manner.
 3. The method of claim 2, wherein the pluralityof terminals in the group use a same slot, wherein a slot comprises asubset of available symbols and available subcarriers in a sector of aterminal.
 4. The method of claim 2, wherein designating the plurality ofterminals comprises: receiving base station reports from multiplesectors, wherein the reports comprise one or more of signal strength andSINR for terminals in the sectors; and analyzing the reports todetermine terminals to designate.
 5. The method of claim 2, whereinhandling communications comprises handling pilot subcarriers in a samemanner as data subcarriers.
 6. The method of claim 1, wherein thecellular communication method comprises an orthogonal-frequency-divisionmultiple-access (OFOMA) method.
 7. The method of claim 1, wherein theplurality of terminals comprises boundary terminals that are on one ormore of a sector boundary and a cell boundary.
 8. The method of claim 1,wherein the plurality of terminals comprises terminals with a relativelylow signal-to-interference ratio (SINR) with respect to other terminalsof the plurality of terminals.
 9. The method of claim 1, wherein theplurality of terminals comprises: distressed terminals with a low SINRrelative to sacrificial terminals; and sacrificial terminals with a highSINR relative to distressed terminals.
 10. The method of claim 1,wherein the plurality of terminals comprises distressed terminals with alow SINR relative to sacrificial terminals, and sacrificial terminalswith a high SINR relative to distressed terminals, the method furthercomprising selecting slot allocations of a sector comprising adistressed terminal to correspond with slots in which other sectors arenot transmitting.
 11. The method of claim 1, wherein the plurality ofterminals comprises a single terminal that has a relatively low SINRcompared to other terminals of the plurality of terminals, the methodfurther comprising: multiple base stations transmitting a same signal tothe single terminal; and the single terminal receiving copies of thesame signal from each of the multiple base stations.
 12. The method ofclaim 11, wherein each of the multiple base stations weights itstransmission by a scaling factor.
 13. The method of claim 11, furthercomprising coordinating beams from two different sectors to the singleterminal.
 14. The method of claim 11, further comprising coordinatingbeams from two different cells to the single terminal.
 15. The method ofclaim 1, further comprising: steering a beam toward a distressedterminal in a serving sector; and steering a beam away from a distressedterminal in an interfering sector.
 16. The method of claim 15, whereinsteering comprises beamforming using multiple antennas from at least oneof a sector and a cell.
 17. The method of claim 15, wherein steeringcomprises beamforming using multiple antennas from across at least oneof one or more sectors and one or more cells.
 18. The method of claim 1,further comprising: performing space-time coding of the data to betransmitted; and transmitting the coded data using available antennas,wherein the available antennas comprise multiple antennas from at leastone of a sector and a cell.
 19. The method of claim 1, furthercomprising: performing space-time coding of the data to be transmitted;and transmitting the coded data using available antennas, wherein theavailable antennas comprise multiple antennas from at least one of oneor more sectors and one or more cells.
 20. Anorthogonal-frequency-division multiple-access (OFDMA) communicationmethod, comprising: identifying a distressed terminal of a plurality ofterminals in a cellular network, wherein the distressed terminal isidentified as receiving downlink transmission signals having an signalto interference and noise ratio (SINR) less than a threshold; andcoordinating downlink transmission to the plurality of terminals basedon at least the identified distressed terminal, wherein the networkcomprises cells, and each cell comprises sectors; wherein coordinatingthe downlink transmission comprises multiple base stations transmittinga same signal to the identified distressed terminal; and the distressedterminal receiving a copy of the same signal from each of the multiplebase stations, comprising selecting pseudo-random bit sequences (PRBS),and aligning the PRBS of the multiple sectors and multiple cells,wherein the aligning comprises assigning a time-frequency slot for theidentified distressed terminals; wherein the plurality of terminalscomprises distressed terminals with a low SINR relative to sacrificialterminals, and sacrificial terminals with a high SINR relative todistressed terminals, the method further comprising: a sectortransmitting data to a terminal in an allocated slot; and pairing adistressed terminal with one or more sacrificial terminals such thattransmit power for the distressed terminal is increased while transmitpower for the sacrificial terminal is decreased; designating adistressed terminal in a serving sector and a distressed terminal in aninterfering sector as being in the plurality of terminals; and usingbeams to reduce interference between the distressed terminal in theserving sector and the distressed terminal in the interfering sector.21. The method of claim 20, wherein coordinating downlink transmissioncomprises: designating the plurality of terminals as a group, wherein aterminal comprises a cellular communication device; and handlingcommunications involving a terminal in the group in a preferentialmanner.
 22. The method of claim 21, wherein the plurality of terminalsin the group will use a same slot, wherein a slot comprises a subset ofavailable symbols and available subcarriers in a sector of a terminal.23. The method of claim 20, wherein the plurality of terminals comprisesboundary terminals that are on one or more of a sector boundary and acell boundary.